Add deregister Command for Cluster Removal

[emreberber]

Description

While sveltosctl provides a register command to add clusters to a Sveltos management namespace, there is currently no deregister command to remove a cluster.

Proposed Solution

Add a sveltosctl deregister command to simplify cluster removal. For example:

sveltosctl deregister cluster \
    --namespace=<namespace> \
    --cluster=<cluster-name>

Benefits

1. Consistency: Provides symmetry with the existing register command.
2. Ease of Use: Simplifies cluster management by reducing reliance on kubectl.
3. Improved User Experience: Allows users to manage cluster lifecycle entirely through sveltosctl.

[gianlucam76]

Will be part of next release (v0.44.0)